Tong-bin Zhao

Case histories of rock bursts under complicated geological conditions

During the past decade’s exploitation of coal seams in Muchengjian Mine in Jingxi Coalfield, there were nearly thirty rock burst events, which hindered the safe and efficient coal production. Two typical mining areas were selected for analysis where almost half of rock burst events occurred. The research was aimed at finding connections between the occurrence of rock bursts and geological characteristics. The temporal and spatial characteristics of rock bursts were described in detail, the geological characteristics were investigated carefully, and the possible reasons for rock bursts were analyzed. The details documented in these cases not only provide an essential reference value for understanding the development mechanism of rock bursts, but also provide a basis for selecting control measures and optimizing related technical parameters during tunneling or mining under complicated geological conditions.

Bolt pull-out tests of anchorage body under different loading rates

Based on the force analysis and mechanical transmission mechanism of grouting bolts, the self-developed test apparatus for interfacial mechanics is used to study the distribution rule of axial force and interfacial stress of bolts in anchorage body. At the same time, pull-out tests of anchorage body are simulated with the particle flow code software , and stress distribution and failure patters are researched under different loading rates. The results show that the distribution of axial force and interfacial shear stress is nonuniform along the anchorage section: axial force decreases, shear force increases first and then decreases, and the maximum value of both of them is closed to the pull-out side; with the increase of loading rates, both of axial force and interfacial shear stress show a trend of increase in the upper anchorage section but changes are not obvious in the lower anchorage section, which causes serious stress concentration; failure strength of pull-out and loading rates show a linear correlation; according to loading rates’ impact on the anchoring effect, the loading rates’ scope can be divided into soft scope (  mm/s), moderate scope (10 mm/s  mm/s).

Effects of Particle Size on Fault Gouge Frictional Characteristics and Associated Acoustic Emission

Our experimental work was designed to explore the particle size effect of simulated fault gouge on slip characteristics by the conventional double-direct shear friction configuration combined with acoustic emission (AE). The following conclusions were drawn: (1) smaller particles allow for an initially higher compaction rate at a higher speed and longer duration for force chain formation and destruction. The larger the particle size is, the higher the slipping displacement rate is; (2) the smaller the particle size is, the larger the friction coefficient is, and thus the higher the fault strength is. In addition, the larger the shear velocity is, the higher the fault strength is; (3) the smaller the particle size is, the higher the shear stress drop generated by the stick-slip is, and the stronger the dynamic slip intensity for a stick-slip period is; and (4) surface defects of forcing blocks possibly help to embed foregoing “stability” and “stable sliding” into the normal stick-slip stage. Especially, the “stable sliding” is possibly related to formation of stubborn force chains. These findings may shed some insights into further clarification of slipping characteristics and discrimination of precursory signs of fault dynamic instability with different-sized gouge particles.

Influence of Fissure Number on the Mechanical Properties of Layer-Crack Rock Models under Uniaxial Compression

Many case studies have revealed that rock bursts generally occur in the high stress concentration area where layer-crack structures often exist, especially for brittle coal or rock masses. Understanding the mechanical properties of layer-crack rock models is beneficial for rational design and stability analysis of rock engineering project and rock burst prevention. This study experimentally investigated the influence of fissure number on the mechanical properties of layer-crack rock models through uniaxial compression tests. The digital speckle correlation method (DSCM) and acoustic emission (AE) techniques were applied to record and analyze the information of deformation and failure processes. Test results show the following: the bearing capacity of layer-crack specimen decreases compared with intact specimen, but their failure modes are similar, which are the splitting failure accompanied with local shear failure; the nonuniform deformation phenomenon begins to appear at the elastic deformation stage for layer-crack specimens; the AE behavior of intact specimens consists of three stages, that is, active stage, quiet stage, and major active stage, but for layer-crack specimens, it is characteristic by three peaks without quiet stage. In addition, as the fissure number of layer-crack specimens increases, the bearing capacity of specimens decreases, the appearing time of nonuniform deformation phenomenon in the specimen surface decreases, the AE events are denser and denser in each peak stage, and the risk of dynamic instability of layer-crack structure increases. At last, the failure mechanism of layer-crack structure and the related mitigation advices were discussed based on the test results. In general, the novelty is that this paper focuses on the failure mechanism of layer-crack structure directly.

Numerical Investigation of Influences of Drilling Arrangements on the Mechanical Behavior and Energy Evolution of Coal Models

Destress drilling method is one of the commonly used methods for mitigating rock bursts, especially in coal mining. To better understand the influences of drilling arrangements on the destress effect is beneficial for rock burst mitigation. This study first introduced the rock burst mitigation mechanism of the destress drilling method and then numerically investigated the influences of drilling arrangements on the mechanical properties of coal models through uniaxial compression tests. Based on the test results, the energy evolution (i.e., the energy dissipation and bursting energy indexes) influenced by different drilling arrangements was analyzed. When the drilling diameter, the number of drilling holes in one row, or the number of drilling rows increases, the bearing capacity of specimens nonlinearly decreases, but the energy dissipation index increases. In addition, the drilling diameter or the number of drilling holes in one row affects the failure mode weakly, which is different from that of the number of drilling rows. Consequently, the bursting energy index decreases as increasing the drilling diameter or the number of drilling holes in one row, but as increasing the number of drilling rows, the variation law of bursting energy index is not obvious. At last, the influencing mechanism of drilling arrangement on the rock burst prevention mechanism of the destress drilling method was discussed and revealed.

Research on shear-slip characteristics with different-size rectangular zigzag gouge by double-direct tests:

We designed and manufactured several types of simulated faults with different-size rectangular zigzag gouges, and sheared them by traditional double-direct shear tests. Simultaneously, the shear force and corresponding acoustic emissions were recorded during shear-slip of rectangular zigzag gouge, and the variations of shear force and acoustic emission energy characteristics related to shear-slip events were analyzed. Especially, the relationship between shear-slip characteristics and zigzag length and width was also revealed. In addition, the significant differences of slip characteristic parameters between shear-slip of an intact zigzag and friction and sliding on rough plane of a sheared zigzag were summarized, and it might be used to judge the different stages of a fault slip with zigzag gouge. In conclusion, this work may reveal the characteristics of shear-slip and friction under conditions of different-size rectangular zigzag gouges of simulated fault and thus provide valuable insights into precursors of earthquake.